1. Field of the Invention
The present invention relates to radiation detectors utilizing scintillators, and, more specifically, it relates to radiation detectors capable of collecting three-dimensional data.
2. Description of the Related Art
Positron emission tomography (PET) is a nuclear medicine imaging technique that has been employed to diagnose cancer, scan brain functions, and the like using images. In PET, a drug labeled with a radioactive isotope that emits positrons is injected into a subject, and two photons (gamma rays) emitted exactly in opposite directions after positron annihilation are simultaneously counted with a detector portion disposed around the subject. Thus, the position of a radiation source is specified to form an image. In the detector portion, detection units each including a photomultiplier tube (PMT) and a scintillator crystal disposed thereon are arranged in a circle. Examples of the scintillator crystal include Bi4Ge3O12 (BGO) and Lu2SiO5 (LSO).
In conventional PET systems, the resolution decreases at the edge of the field of view. As shown in FIG. 1, gamma rays 11 generated at the center of the field of view (circle) are perpendicularly incident on scintillator crystals 12, whereas gamma rays 13 generated at an edge of the field of view are obliquely incident on the scintillator crystals 12. This causes a detection position error (parallax error) due to the length (height) of the scintillator crystals. To counter this, DOI-PET, a technique for suppressing the parallax error by identifying the information of the light-emitting position in the scintillator in the depth direction (depth of interaction; DOI) is drawing attention as a next-generation technique.
Japanese Patent Application Publication No. 2006-522925 (JP 2006522925), also published as WO 2004090572, discloses a method of identifying depth of interaction (DOI), in which a continuous scintillator crystal is disposed on a position-sensitive PMT (PS-PMT). In JP 2006522925, DOI is calculated from the difference in the diffusion of the scintillation light according to the depth of the light-emitting position. Another known method uses multi-layer detection units in which a plurality of scintillator crystals are disposed on a PS-PMT, on which layers of the scintillator crystals are stacked. Japanese Patent Laid-Open No. 11-142524 (JP 11142524) discloses an exemplary method in which DOI is identified by controlling the distribution of scintillation light with the presence/absence and positions of reflective members disposed between the scintillator crystals.
However, in a method using a continuous scintillator crystal, because the scintillation light is propagated isotropically in the crystal, the scintillation light is diffused over a wide area in the PS-PMT. Therefore, to identify the difference in the diffusion of the scintillation light according to the difference in depth of the light-emitting position, information has to be collected from a large number of detection pixels in the PS-PMT. If the depth of the light-emitting position is estimated by the calculation of a few pixels, the error increases.
In contrast, with the method in which layers of scintillator crystals are stacked, the depth of the light-emitting position can be estimated by the calculation of a few pixels. However, a very large number, for example, 120,000 scintillator crystals are required for each system. In addition, because reflective members have to be selectively disposed between the scintillator crystals, a process of assembling the detection units is very complex.